Update README.md

README.md

 
 ## Performance Tips
 
-**Read Stream Event Overheard**
+**Read Stream Event Overhead**
 
 When reading data via `fs.readStream` the process seems blocking JS thread when file is large, it's because the default buffer size is quite small (4kb) which result in large amount of events triggered in JS thread, try to increase the buffer size (for example 100kb = 102400) and set a larger interval (which is introduced in 0.9.4 default value is 10ms) to limit the frequency. 
 
-**Reduce RCT Bridge and BASE64 Overheard**
+**Reduce RCT Bridge and BASE64 Overhead**
 
 React Native connects JS and Native context by passing JSON around React Native bridge, and there will be an overhead to convert data before they sent to each side. When data is large, this will be quite a performance impact to your app, it's recommended to use file storage instead of BASE64 if possible.The following chart shows how much faster when loading data from storage than BASE64 encoded string on iphone 6.